1. Field of the Invention
The present invention relates to butterfly type control valve, comprising a body in which is provided a housing containing a mobile plug called a butterfly, this plug rotating about an axis; said housing being extended on either side by an inlet channel and by an outlet channel which are respectively arranged to be connected to an inlet conduit and an outlet conduit, said inlet channel, said housing and said outlet channel defining a connection passage linking the inlet conduit to the outlet conduit, the effective cross section of this passage and therefore the flow rate of the fluid flowing through the valve being controlled according to the angular position of the plug in the housing.
2. Discussion of Background
The butterfly valves which are currently used for fluid control purposes, comprise a body in which is provided a housing containing a plug which rotates about a central axis. This housing is extended on one side by an inlet channel and on the other side by an outlet channel, arranged respectively to be connected to an inlet conduit and to an outlet conduit (not shown). When the plug rotates about its axis of rotation, the size of the passage connecting the inlet channel to the outlet channel through the valve's housing is increased or reduced. Thus by rotating this plug, the flow of the fluid flowing through the valve can be controlled.
This type of valve unfortunately presents a number of drawbacks. When the opening starts, the fact that the passage between the inlet channel and the outlet channel is small means that the fluid flowing through the valve does so at high speed. The position of the mobile plug is then unstable which makes good control difficult.
Furthermore, the static pressure on the mobile plug is not symmetric in relation to its axis of rotation. The result is a torque which tends to make it rotate about its axis which makes fluid control erratic.
Lastly, the dynamic torque on the mobile plug depends on the opening angle and the differential pressure, which also leads to unstable control.
An answer to some of these problems was provided by a butterfly valve which comprises a back pressure baffle placed in its housing, in the vicinity of the outlet channel. This baffle reduces the dynamic torque which makes it easier to rotate the plug and to keep it in position.
This valve does still however present some drawbacks. The back pressure baffle is only efficient for a given flow rate and is not able to overcome the above-mentioned problems which are posed when the valve starts to open. It does not make it possible either to ensure a stable control of the flow rate when slightly open.
Other solutions, such as those illustrated in the French publication FR-A-2 616 874 and the European publication EP-A-0 188 130, have been provided by fitting an element which generates a drop in pressure located in the valve's opening and closing zones. In the first publication, this element comes in the form of perforated circular sector the center of which is that of the plug's axis. In the second publication, the element generating a drop in pressure is made up of one or several perforated plates arranged parallel to one another. In these two forms, when the plug is opened slightly, the flow of fluid runs on both opposite sides of the plug, which prevents the flow rate from being precisely controlled when it is low.